In a gas- and steam-turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is utilized for generating steam for the steam turbine. The transfer of heat is effected in a waste-heat steam generator which is disposed downstream of the gas turbine and in which a number of heating areas for preheating water, for generating steam and for superheating steam are normally disposed. The heating areas are connected to the water/steam circuit of the steam turbine. The water/steam circuit normally comprises several, e.g. three, pressure stages, whereby each pressure stage may have an evaporator heating area.
For the steam generator disposed as a waste-heat steam generator downstream of the gas turbine on the heating-gas side, a number of alternative configuration concepts are suitable, namely the configuration as a once-through steam generator or the configuration as a circulation steam generator. In the case of a once-through steam generator, the heating of steam-generator tubes provided as evaporator tubes leads to evaporation of the flow medium in the steam-generator tubes in a single pass. In contrast, in the case of a natural- or forced-circulation steam generator, the water in circulation is only partly evaporated when passing through the evaporator tubes. After separation of the generated steam, the water that is not evaporated in the process is fed again to the same evaporator tubes for further evaporation.
A once-through steam generator, in contrast to a natural- or forced-circulation steam generator, is not subject to any pressure limitation so that it can be configured for live-steam pressures well above the critical pressure of water (PCri ≈221 bar), where no differentiation between the water and steam phases and therefore also no separation of the phases is possible. A high live-steam pressure promotes a high thermal efficiency and thus low CO2 emissions in a fossil-fired power station. In addition, a once-through steam generator has a simple type of construction compared with a circulation steam generator and can therefore be manufactured at an especially low cost. The use of a steam generator configured according to the once-through principle as a waste-heat steam generator of a gas- and steam-turbine plant is therefore especially favorable for achieving a high overall efficiency of the gas- and steam-turbine plant in a simple type of construction.
Particular advantages with regard to the manufacturing cost as well as with regard to required maintenance work are offered by a waste-heat steam generator with a horizontal type of construction in which the heating medium or heating gas, i.e. the waste gas from the gas turbine, is conducted in an approximately horizontal direction of flow through the steam generator. In a steam generator with a horizontal type of construction, the steam-generator tubes of a heating area may, however, be exposed to widely differing heating, depending on their positioning. Particularly where steam-generator tubes of a once-through steam generator are connected on the outlet side to a common collector, different heating of individual steam-generator tubes may lead to the merging of steam flows having steam parameters differing greatly from one another and thus to undesirable efficiency losses, in particular to a comparative reduction in the effectiveness of the heating area concerned and consequently to a reduction in steam generation. Different heating of adjacent steam-generator tubes, particularly in the region where they lead into collectors, may also result in damage to the steam-generator tubes or the collector. The inherently desirable use of a once-through steam generator with a horizontal type of construction as a waste-heat stem generator for a gas turbine can thus bring with it considerable problems with regard to an adequately stabilized flow.
A steam generator is known from EP 0 944 801 B1 which is suitable for a configuration in a horizontal type of construction and which also has the aforementioned advantages of a once-through steam generator. To this end, the evaporator heating area of the known steam generator is connected up as a once-through heating area and configured such that a steam-generating tube which is heated to a greater extent compared with a further steam-generator tube of the same once-through heating area has a higher throughput of the flow medium than the further steam-generator tube. Here, the expression once-through heating area refers in general to a heating area that is configured for a through flow according to the once-through principle. The flow medium fed to the evaporator heating area connected as a once-through heating area is thus completely evaporated in a single pass through this once-through heating area or through a heating-area system comprising a plurality of once-through heating areas connected one behind the other.
The evaporator heating area of the known steam generator which is connected up as a once-through heating area thus exhibits, in the type of flow characteristic of a natural-circulation evaporator heating area (natural-circulation characteristic) where different degrees of heating of individual steam-generator tubes occurs, a self-stabilizing behavior which leads, without the need for external influence to be exerted to a balancing of the temperatures on the outlet side even in steam-generator tubes heated differently and connected in parallel on the flow-medium side.
The known steam generator has an evaporator system with a multi-stage design, in which a further evaporator/once-through heating area is connected downstream of a first once-through heating area on the flow-medium side. In order to ensure a reliable and comparatively homogeneous overflow of the flow medium from the first to the second once-through heating area, the known steam generator is fitted with a complex distributor system which requires a comparatively high outlay in terms of construction and design.